A real time non-invasive monitoring system for detection of hypovolemic state using RR interval variability in very low frequency ranges

In offline analysis, very low frequency variability in RR interval and systolic arterial pressure has been reported below 0.1 Hz during head-up tilt (HUT) tests in conditions similar to hypovolemic states in humans. We designed a real time and non-invasive monitoring system of very low frequency RR interval variability to detect the hypovolemic state resulting from internal and external hemorrhages. Eight male Sprague-Dawley rats were subjected to monitoring before (normal state) and after withdrawal of 2 ml blood/100 g body weight over 15 minutes (hypovolemic state). Using one-channel electrocardiogram (ECG), the detection system monitored in real time the very low frequency components of RR interval variability using Fast Fourier Transform (FFT). Temperature mappings of rat abdomen were conducted simultaneously to monitor the hypothermic state after blood withdrawal using a local temperature mapping system with a deep body thermometer. This system demonstrated oscillation of the RR interval at 0.075±0.015 Hz in real time after blood withdrawal. The deep body temperature decreased significantly from 37.4±0.9 degrees centigrade to 35.5±1.2 degrees centigrade (p<0.05) within 1 hour after blood withdrawal. Our monitoring system appears promising for the detection of hypovolemic state resulting from massive hemorrhage using a one-channel ECG monitor.